Semiconductor substrates are used in a wide variety of applications. For example, semiconductor substrates having a slot formed therethrough may be used to provide ejection actuators for micro-fluid ejection heads. Slots in a silicon wafer containing multiple substrates may be formed by a variety of micro-machining techniques including, but not limited to, sand blasting, chemical wet etching, dry etching, laser cutting, and the like. Deep reactive ion etching (DRIE) of slots through silicon wafers offers significant advantages over other technologies such as grit blasting, laser cutting, and wet etching. Specifically, the DRIE process provides better dimensional control of the topside and backside openings compared to the grit blasting process. In addition, chips with a slot formed by the DRIE process are approximately 5 times stronger in 3-point bend testing than grit blasted chips.
When compared to wafers that have slots etched therethrough using KOH or TMAH in a wet chemical process, wall angles of the slots may be much smaller using the DRIE process. For example, re-entrant wall angles between 0° and 8° may be achieved with a DRIE process whereas wet processes produce wall angles of 54.7° in <100> silicon. Hence, DRIE processes for slots may provide a more efficient use of silicon.
The DRIE process for etching slots through a thickness of a semiconductor substrate wafer includes a series of sequential steps of alternating etching and passivation. Such dry etching techniques are described in U.S. Pat. Nos. 5,611,888 and 5,626,716 to Bosch et al., the disclosures of which are incorporated herein by reference.
During deep reactive ion etching (DRIE) of semiconductor wafers, the wafers are positioned and held in place on an electrostatic clamping disk. The clamping disk uses a DC power source to induce a charge on the surface of the wafer to be etched. The charge on the wafer provides an electro-static force that pulls the wafer onto the clamping disk. As opposed to mechanical clamping, an electrostatic clamping mechanism increases a surface area of the wafer available for etching.
The clamping disk also includes a cooling mechanism for cooling a back side of the wafer during the DRIE etching process. The cooling mechanism provides helium gas which flows through channels in the clamping disk to the back side of the wafer. Helium gas serves as a heat transfer medium between the wafer and the clamping disk.
If DRIE etching of the wafers is used to form slots through a thickness of the wafers, an etch stop material must be used on the back side of the wafers to protect the clamping disk from an etching plasma generated during the DRIE process and to prevent escape of helium gas used to cool the back side of the wafers. The escape of helium gas can cause inadequate cooling of the wafers during the etching process, and/or the wafers may be pushed off of the clamping disk by an increase in helium pressure to compensate for helium gas escape.
Various etch stop materials may be applied to the back side of the wafers to protect the clamping disk from damage and to prevent the escape of helium gas. Of the etch stop materials that may be used, relatively hard etch stop materials provide the best protection for the clamping disk. However, relatively hard etch stop materials are difficult to completely remove from the back side of the wafers once the etching process is complete.
Relatively, soft etch stop materials, such as photoresist polymers, are easier to remove from the back side of the wafers. However, under etching conditions, degradation products or a film residue from the relatively soft etch stop materials may accumulate on the clamping disk making it difficult to adequately clamp and seal the wafers on the clamping disk. The residue may also interfere with the flow of cooling gas to the back side of the wafers and may reduce thermal conduction between the wafers and the clamping disk. Removal of the residue may require significant downtime for the etching system thereby reducing product yield. Accordingly, improved methods for DRIE etching of wafers are needed to improve product yield and reduce problems associated with use of apparatus for DRIE etching of wafers.
With regard to the foregoing, the disclosure provides a process for etching semiconductor substrates using a deep reactive ion etching process to produce through holes or slots (referred to hereafter collectively as “slots”) in the substrates (which may be collectively part of a wafer of semiconductor substrates). The process includes applying a first layer to a first surface of substrate to provide an etch mask material layer on the first surface of the substrate. A second layer is applied to a second surface of the substrate to provide an etch stop material layer on the second surface of the substrate. The first layer and the second layer have similar solubilities in one or more organic solvents. The substrate is etched from the first surface of the wafers to provide a slot in the substrate. After etching the substrate, the etch mask material layer and the etch stop material layer are removed by contacting the first surface and the second surface of the substrate with a single organic solvent.
Advantages of the exemplary embodiments described herein may include the ability to protect semiconductor substrate during a dry etching process with materials that are easily removed once the etching process is complete. Since the etch mask material and the etch stop material are soluble in a single organic solvent, the process for removing the etch mask and etch stop from the substrate is substantially simplified since only one substrate solvent contact step is required. Another advantage is that a wider variety of etch stop materials may be used that have an increased resistance to residue formation on the clamping disk thereby reducing interference in the flow of cooling gas to the back side of the substrate and improving a seal between the substrate and the clamping disk.